igneous
Iron-rich basalt or weathered volcanic rock
Basalt, likely oxidized mafic volcanic rock; composition mainly plagioclase feldspar, pyroxene, and iron oxides
AI-generated identification · may be incorrectDark brown-black, fine-grained and massive, with dull to weakly vitreous luster and irregular fracture; typical hardness is about 6 Mohs and specific gravity about 2.8–3.1. The rusty reddish-brown areas indicate surface oxidation of iron-bearing minerals.
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Physical properties
Dark brown-black, fine-grained and massive, with dull to weakly vitreous luster and irregular fracture; typical hardness is about 6 Mohs and specific gravity about 2.8–3.1. The rusty reddish-brown areas indicate surface oxidation of iron-bearing minerals.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic episodes; weathering produced the brown oxidation rind. The photograph does not show diagnostic crystals or vesicles, so exact identification is approximate.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, and building material; attractive polished pieces may be collected, but this rough specimen has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant magnesium- and iron-rich minerals.
Field identification & locations
Look for a dense, fine-grained dark rock that does not fizz with dilute hydrochloric acid and may attract a weak magnet if magnetite-rich. Common worldwide, especially in volcanic regions and oceanic crust.
